1. Field of the Invention
The present invention relates to apparatus for non-invasively measuring one or more blood parameters. More specifically, the invention relates to apparatus for the transcutaneous measurement of vascular access blood flow. The invention can also be used for precise access location, as a “flow finder,” and also can be used to locate grafts and to localize veins in normal patients for more efficient canulatization.
2. Related Art
Routine determination of the rate of blood flow within the vascular access site during maintenance hemodialysis is currently considered an integral component of vascular access assessment. While the relative importance of vascular access flow rates and venous pressure measurements in detecting venous stenoses is still somewhat controversial, both the magnitude and the rate of decrease in vascular access flow rate have been previously shown to predict venous stenoses and access site failure. The traditional approach for determining the vascular access flow rate is by Doppler flow imaging; however, these procedures are expensive and cannot be performed during routine hemodialysis, and the results from this approach are dependent on the machine and operator.
Determination of the vascular access flow rate can also be accurately determined using indicator dilution methods. Early indicator dilution studies determined the vascular access flow rate by injecting cardiogreen or radiolabeled substances at a constant rate into the arterial end of the access site and calculated the vascular access flow rate from the steady state downstream concentration of the injected substance. These early attempts to use indicator dilution methods were limited to research applications since this approach could not be routinely performed during clinical hemodialysis. It has long been known that in order to determine the vascular access flow (ABF) rate during the hemodialysis procedure, the dialysis blood lines can be reversed (by switching the arterial and venous connections) to direct the blood flow within the hemodialysis circuit in order to facilitate the injection of an indicator in the arterial end of the access site and detect its concentration downstream (N. M. Krivitski, “Theory and validation of access flow measurements by dilution technique during hemodialysis,” Kidney Int 48:244-250, 1995; N. M. Krivitski, “Novel method to measure access flow during hemodialysis by ultrasound velocity dilution technique,” ASAIO J 41:M741-M745, 1995; and T. A. Depner and N. M. Krivitski, “Clinical measurement of blood flow in hemodialysis access fistulae and grafts by ultrasound dilution,” ASAIO J 41:M745-M749, 1995)). D. Yarar et al., Kidney Int., 65: 1129-1135 (1999), developed a similar method using change in hematocrit to determine ABF. Various modifications of this approach have been subsequently developed. While these latter indicator dilution methods permit determination of the vascular access flow rate during routine hemodialysis, reversal of the dialysis blood lines from their normal configuration is inconvenient and time-consuming since it requires that the dialyzer blood pump be stopped and the dialysis procedure is relatively inefficient during the evaluation of the flow rate which can take up to twenty minutes. Furthermore, some of these indicator dilution methods also require accurate determination of the blood flow rate.
Clinical usefulness and ease of use are major developmental criteria. From a routine clinical point of view the need to design a simple sensor, easily attached to the patient, requiring no line reversals, no knowledge of the dialysis blood flow rate, Qb, and transcutaneously applied to skin, thereby accomplishing the measurement within a total of 1-2 minutes, is crucial to have repeated, routine meaningful ABF trend information, whereby access health is easily tracked.